1. Field of the Invention
The invention relates generally to reverse osmosis and ultra filtration fluid separation processes, and is applicable particularly to water desalination and purification by reverse osmosis.
2. Prior Art
In reverse osmosis filtered sea water, eg saline water, is pumped to a working pressure well in excess of osmotic pressure and is fed into a pressure vessel containing semi-permeable membranes. Purified product fluid, i.e. potable water, permeates through the membranes, and concentrated brine remaining behind the membrane at nearly full working pressure is exhausted to prevent excessive salt concentration. The concentrate fluid must first be depressurized by a throttle valve which wastes the pressure energy, or by a mechanical energy recovery device which permits recovery of energy in the concentrate fluid.
Two examples of energy recovery devices in reciprocating pumps are shown in U.S. Pat. No. 3,558,242, inventor W. D. Jenkyn-Thomas, and U.S. Pat. No. 4,124,288, inventor L. P. S. Wilson. In these patents, a sliding spool valve or rotary valve is used to control direction of fluid flow into and out from the expansion chamber of the feed pump, the valves being phased relative to the pump stroke to permit fluid entry and exit precisely at extreme limits of the piston stroke. This, of course, requires accurate valve timing and fast valve shift to reduce chances of hydraulic lock or system pressure loss, because movement of the valve means cannot occur while the piston means is moving. It follows that the valve timing mechanism requires close manufacturing tolerances, and thus excessive wear in the valve mechanism would likely cause severe problems.
The working pressure is commonly of the order of 70 Kg/sq. cm. and thus directional valve gear is subject to relatively high forces, particularly in view of the relatively poor lubricity of sea water. In the two references referred to above, directional valve shifting occurs across ports subjected to high pressure difference which subject the valves to severe erosion problems, and may impose high loading on the valve gear, tending to aggravate wear and compound the risk of pressure loss.
U.S. Pat. No. 4,187,173, issued to the present inventor, discloses internal dwell means which permit approximate equalization of pressure between pumping and expansion chambers of the feed cylinder and approximate equalization of pressure across ports of the main directional valve prior to opening of such ports. Applicant discloses in FIG. 5 of his patent, a crank driven "yieldable piston," in which there is axial movement between portions of the piston and the piston rod so that the piston rod stroke is longer than the piston stroke. Axial movement between the piston and piston rod provides a dwell interval which enables transfer of fluid volume by independent movement of the piston permitting approximate equalization of pressure across the chambers of the feed pump. During dwell the piston rod travels by itself and acts as a plunger producing a relatively slow pressure change. This is because the relatively small rod diameter is moving within the relatively large trapped fluid volume and excessive pressure rise or cavitation is prevented by check valves controlling fluid flow to and from the pumping chamber.